Electrolytic device



March 12, 1940. J, BURNHAM 2,193,711

ELECTROLYTIC DEVICE Filed Dec. 14, 1957 INVENTOR. JOHN BURNHAM BY WIQQXWATTORNEYS meme Mn. 12, 1940 UNITED STATES PATENT OFFICE r 2,193,111mo'mopmc mrvrca John Bnrnham. North Adamsrlllasa, assignor to SpragueSpecialties Company, North Adams, Mass, a corporation of MassachusettsApplication December 14, 1937, Serial No. 179,778

' 4 Chims. (01. 148-8) The present invention relates to electrolyticdevices, and more particularly to electrolytic condensers and theirmanufacture.

Electrolytic condensers in their usual form comprise two electrodes, atleast one of which is of so-called filming metal, and is providedusually with an electrolytically-formed film, formed of the partlyhydrated oxide of the metal of the electrode. The condenser is providedwith a suitable film-forming electrolyte, which may be highly fluid, asin the case of so-called "we electrolytic condensers, or may be more orless viscous, as in the case of dry" electrolytic con-- densers. Myinvention applies to both wet and dry types of electrolytic condensers,and irrespective of whether one or both of the electrodes are filmed.

As a material for the filming electrode, aluminum is usually used and myinvention particularly relates to aluminum electrodes.

To obtain a film of the most favorable properties the aluminum used forthe filmed electrode should be of high purity. Impurities in thealuminum impair the quality of the film for several reasons, of whichthe following two are the most important.

First, as the impurities are generally nonfilming materials, no film isformed on the spots where they appear in or on the surface of thealuminum. This results in a porous film, which increases the leakage andpower factor of the condenser.

Secondly, most of the impurities, under the action of the electrolyte ofthe condenser are subject to electro-chemical reaction, which causeslocal damage to the film, thereby reducing the useful life of thecondenser.

Films formed on electrolytically refined aluminum having a puritygreater than 99.9% and usually referred to as Hoopes aluminum. are foundto be of much better quality than films formed on aluminum of commercialpurity. In practice, however, with unetched electrodes, the

' difference is not of sufiicient-commercial importance to warrant theincreased cost due to the higher price of Hoopes aluminum.

However, if the electrodes are to be chemically etched to increase theirsurface area and thus to correspondingly increase the capacity of thecondenser, the situation changes considerably. First, the deleteriousinfluence of impurities may be accentuated with etched surfaces, andsecondly, the cost of the aluminum plays a much smaller part in the costof the condenser.

Thus it would seem highly advantageous to use Hoopes aluminum for etchedelectrodes. This, however, could not be achieved so far because Hoopesaluminum did not lend itself to etching by known processes.

That Hoopes aluminum could not be etched in solutions and by processeswhich have been found satisfactory for the etching of aluminum ofcommercial purity, seems to be due to the following:

In aluminum of commercial purity the etching is produced primarily bytwo simultaneous reactions. One is due to the metallic impuritiespresentin the aluminum which set up minute voltaic cells which in the presenceof the acid solution rapidly etches the surface of the aluminum. Thesecond is due to actionof the hydrogen ions of the acid.. The voltaicaction is the more important and accounts for the major part of theetching. The attack by the hydrogen ions in this reaction can beconsidered negligible.

In Hoopes aluminum, however, the absence of the metallic impurities, andthe voltaic cells formed thereby, slows down the etching process to analmost negligible extent.

Furthermore, assuming the same heat treatment and working, the crystalsize of the Hoopes aluminum is considerably larger than that of'the lesspure aluminum, and as the etching takes place primarily at theinter-crystalline boundaries, and these are few in Hoopes aluminum, theattack by the etching solution of the Hoopes aluminum is practicallynil. I

The object of the present invention is to provide a novel etchingsolution and etching process which permits the etching of the Hoopesaluminum, and in general of high purity aluminum, and achieves this inan economical way.

According to the invention I provide an etching solution which comprisesan etching acid and a positive catalyst added thereto, which catalyst iseffective in increasing the attack of the hydrogen ions of the acidsolution.

Such a catalyst is a salt and should preferably be a salt which ishighly soluble in the etching acid solution. As a rule I prefer to usean aluminum salt, for example, aluminum chloride or aluminum sulphate,aluminum nitrate, etc.

-Salts of other metals which are lower in the electromotive series thanaluminum may also be used, for example, salts of copper or iron, such astheir chlorides, are suitable. However, when using salts of other metalsthan aluminum there v num of lower purity has been subjected to etchingin a hydrochloric acid solution, some aluminum chloride was alsoproduced during the etching reaction. However, if Hoopes aluminum isplaced in a similar hydrochloric acid solution, the etching reactiondoes not start and no aluminum chloride is formed, nor would the smallamount of aluminum chloride incidentally formed in the aforementionedreaction be sufficient to help to any important extent the etching ofHoopes aluminum.

To perform the etching process of my invention in a reasonably shorttime, the etching solution should be maintained at elevatedtemperatures, preferably between 50 and 85 C.

I have also found that to obtain the best results, definite relationshave to exist between the concentration of the etching acid, the amountof catalyst contained therein, the temperature of the solution, and theduration of the etching. While a wide range of concentrations of theetching acid have been found suitable, as a rule it is necessary thatirrespective of its concentration, it should be saturated orsubstantially saturated with the catalyst.

In the drawing forming part of the specification:

Figure l is a partly sectionized side viewof a wet electrolyticcondenser embodying the invention;

Fig. 2 is a partly sectionized side view of a dry electrolytic condenserembodying the invention;

Fig. 3 is an enlarged section of a portion of an aluminum electrodeetched in accordance with the invention.

Referring to Figure l, the condenser there shown is a'D. C. condenser ofthe type used in the filter circuits of radio receivers. The condensercomprises a cylindrical metallic container I, which preferablyconstitutes the cathode of the condenser, and which may be either offilmforming or non-film-forming metal. The container I is provided witha reduced tubular extension 3, threaded at 4 and extending into afurther reduced tubular portion 5.

Disposed within the container I is an anode assembly 6 comprising acorrugated thin 'foil 1 of Hoopes aluminum etched in accordance with theinvention. The foil 1 is secured to an anode 'riser 8 which serves as asupport and terminal therefor.

The top of the container is provided with a crowned metal cap 9 whichfits into the container end 2. A vent gasket ID of suitable material,for example of wax-impregnated cloth, is interposed between thecontainer l and the cap 9. The free edge of cap 9, with the gasket Illinterposed, is

crimped over a rim provided on the container I to form a liquid-tightseal.

To prevent the anode foil 1 from contacting the container I, there isinterposed between same and the anode assembly 6 an insulator, forinstance, a thin hard rubber cylinder II. To permit free circulationbetween cathode and anode, the insulator II is provided withperforations I2 throughout its area.

The threads 4 on the reduced neck portion 3 are adapted to receive a nut(not shown) by means of which the condenser may be secured to a suitablebase.

The extension has preferably the same inside diameter as the extension3, so that the two form a common bore II. The wall of the extension 5is, however, preferably much thinner than the wall of extension 3.

Placed in the bore I3 is a tightly-fitting plug it preferably ofsemi-hard vulcanized rubber through which is inserted the anode riser 8.

The plug it is preferably of such length as to extend on both sides ofthe neck portion 5. Proper sealing between the plug it and the neck 5may be obtained by fluting the neck portion.

, A suitable film-maintaining electrolyte I5 is provided in thecontainer and fills the container close to its top.

The electrolyte i5 is preferably an aqueous solution of a weak acid,forinstance of boric, phosphoric, citric acid, etc., to which may beadded a salt of a weak acid, which salt does not need to be that of theacid used in the electrolyte.

Fig. 2 illustrates a dry condenser embodying the invention. Thecondenser comprises two electrodes 20 and 2|, which are usually both ofaluminum, although in case of D. C. condensers only one electrode (theanode) needs to be provided with a dielectric film. The filmed electrodeconsists of Hoopes aluminum and is etched in accordance with theinvention. The electrodes are assembled into a roll with theinterposition of spacers 22-22, which are preferably of an absorbentmaterial, such as gauze, paper, Cellophane, or a combination of same,and which serve as carrier for an electrolyte 23.

The electrolyte 23 may be more or less viscous and its ionogens areusually of the same types as have been enumerated above in connectionwith the wet condensers; however, the solvent of the electrolyte usuallycomprises a polyhydric alcohol,

such as glycerol, ethylene glycol, etc., and the electrolyte may alsocontain substances which increase its viscosity and/or conductivity.

The assembled condenser roll is disposed in a container 25, which may beof metal or insulating material. A layer 26 of a suitable sealingcompound serves to seal the container and protect the condenser frommoisture. Terminal wires 21 and 28 of the electrodes 20 and 2|respectively,

pass through the sealing layer 26 to serve as outside connections forthe condenser.

As has been stated before, the filmed electrode or electrodes are madeof Hoopes aluminum and are etched in accordance with the invention.

For this purpose the aluminum prior to its etching is cleansed in knownmanner, for example by dipping it for about 15 seconds in a causticsolution comprising 5% sodium hydroxide heated to about C. Thiscleansing removes all traces of grit grease and other extraneousimpurities that may adhere to the surface of the aluminum.

Following this, the electrode is rinsed with distilled water to removeall traces of the caustic solution.

The electrode is then subjected to the etching, whereby in accordancewith the invention, a suitable catalyst is added to the etching acidsolution prior to the etching and in an amount sufficient to saturate orsubstantially saturate the solution. Suitable catalysts are the salts ofthe type previously mentioned, which as stated, should be highly solublein the etching solution so as to be present therein in large amounts.

As a rule I prefer to use hydrochloric acid solutions to which I add asuilicient amount of aluminum chloride to saturate the solution. Theacid concentration may be selected within wide limits; it may be aconcentration as low as 3%, or it may be a practically saturatedsolution of about 35%.

Too high concentration of the hydrochloric acid solution is, however,preferably avoided as with increasing concentration of the acid thesolubility of the aluminum chloride decreases, so that a 35%hydrochloric acid solution becomes saturated with about .05% aluminumchloride, which is insunicient to get the full benefit of my invention.

Too low acid concentration (below 5%) of the etching solution is alsoobjectionable as it reduces the etching ratio.

In practice I therefore prefer to use concentrations of 5% to 25% (acidby weight).

To a 5% hydrochloric acid solution I add about 25% aluminum chloride,whereas to a 25% acid solution I add about 6.5% aluminum chloride. Inboth cases I obtain substantial saturation.

I found that when using hydrochloric acid as etching acid and aluminumchloride as catalyst, the sum of the concentrations of the hydrochloricacid and of the aluminum chloride in the solution is to be between about30-35% (by weight) to obtain the best etching results.

The etching time usually also depends on the temperature of the etchingsolution. For example, ,with an etching solution comprising 18% ofhydrochloric acid and 12% of aluminum chloride, and maintained at atemperature of to C., the etching time will be about 5 to 7 minutes. Byraising the temperature of the solution to about to C. this time can bereduced to 3 or 4 minutes.

Too high etching speeds are as a rule undesirable because they reducethe etching ratio.

After their etching the electrodes are washed in distilled water toremove all traces of the etchitng solution and then subjected to filmformaion.

In forming the dielectric film on the electrode, various knownfilm-forming electrolytes may be used. For instance, in case of wetcondensers suitable for 550 volts peak voltage, I may use an electrolytecomprising grams of boric acid per liter of distilled water.

While I have described my invention in connection with specificembodiments and by specific examples, I do not wish to be limitedthereto, but desire the appended claims to be construed as broadly aspermissible in view of the prior art.

What I claim is: i

1. In the manufacture of electrodes for electrolytic condensers, theprocess which comprises the steps of adding aluminum chloride in aquantity sufllcient to saturate an aqueous solution of hydrochloricacid, and subjecting aluminum containing less than .1% impurities toetching in said solution.

2. In the manufacture of electrodes for electrolytic condensers, addingto an 18% solution of hydrochloric acid 12% by weight of aluminumchloride, and etching in said solution aluminum having less than .l%

impurities.

between 15 to 35%.

JOHN BURNHAM.

the process which comprises,-

